(a) Field of the Invention
The present invention relates to an objective lens system for endoscopes.
(b) Description of the Prior Art
As one of the conventional objective lens systems for endoscopes, there is already known the vari-focal optical system disclosed by the specification of U.S. Pat. No. 3637282. This optical system is so adapted as to form images of an object OB at different magnification levels on the end surface of incidence of an image guide fiber bundle 1, as shown in FIG. 1, by arranging a telecentric type of master lens consisting of a lens OL and an aperture stop S located in the vicinity of the front focal point thereof before the image guide fiber bundle 1, and further arranging, in a mutually replaceable mode, attachment lenses of a tele type of lens system consisting of a lens AF and lens AL, and a reverse tele type of lens system consisting of a lens AF' and a lens AL' before said master lens.
In an objective lens system having the composition described above, aberrations are corrected favorably in the master lens itself in most cases, and it is necessary to suppress aberration production in the attachment lenses arranged before the master lens so that aberrations are corrected favorably in the objective system as a whole.